1. Field of the Invention
The present invention relates to digital servo systems and more specifically to track seeking and centering servo systems of the type used in disk drives.
2. Description of the Prior Art
It is well known in the field of disk drive design that seek time, which is the time it takes to move a transducer in a disk drive (or in other moving media types of storage devices) from one track to another, may be minimized by using a so-called "bang-bang" technique. The transducer is sped at maximum acceleration from an initial track toward a desired track for half the seek time and is slowed at maximum deceleration for the other half of the seek time. This maximum-acceleration/maximum-deceleration strategy enables the transducer to reach a desired track in the least time possible. But such high speed positioning of the transducer tends to be relatively coarse (imprecise). Accordingly, when the transducer approaches the desired track, it is common practice to activate a track-centering servo system which brings the transducer into finer alignment with the center of the desired track and which holds the transducer at track center.
In one method for aligning the transducer with track center, a first burst of periodic servo data (referred to as the "A burst") is recorded (embedded) exclusively on one side of track center and a second burst of periodic servo data (which is spaced apart from the first burst and is referred to as the "B burst") is recorded (embedded) exclusively on the other side of track center. As the disk spins, the track-centering servo receives disk signals from the transducer, extracts portions of the received disk signals corresponding to the prerecorded A and B bursts, generates an error signal whose magnitude is a function of the difference in amplitude between the transducer-detected A and B burst signals, and attempts to move the transducer in a fashion that forces the magnitude of the error signal to converge to a minimum.
If the transducer detects A and B burst signals of equal amplitudes, the track-centering servo system can assume that equally wide parts of the transducer overlap equally wide regions of the recorded A and B bursts, and as a consequence, that the transducer is precisely aligned with track center. When the amplitudes of each of the detected A and B bursts are nonzero, and the amplitude of one of the detected A and B burst signals is found to be larger than that of the other, the track-centering servo system can decide that the transducer is off track-center and can generate a position correction signal which is proportional to the error signal for moving the transducer into alignment with track center. This correction signal is supplied to a transducer moving means to move the transducer in a manner which ultimately causes the error signal to converge to a minimum.
A problem develops when the A and B burst signal amplitudes are unequal, but one of them is zero. This condition creates a break (nonlinearity) in the relationship between the error signal and the physical misalignment distance (position error). It is desirable to continuously maintain a relatively proportional or linear relationship between the magnitude of the error signal (the difference in amplitude between the detected A and B bursts) and the transducer-to-track-center position error so that the track-centering servo can bring the transducer into alignment with track center in minimal time. The continuity of this relation can be undesirably broken within the track seek/alignment process if, at the end of the maximum deceleration phase of a track seek operation, the transducer comes to rest in a first region which is located approximately midway between two tracks (this first region may be described as a "nonlinear zone") rather than in a second region which is located at the center of a single track (this second region may be referred to as a "linear zone").
In such a situation, the detecting portion of the transducer; which is usually narrower than the physical width of either of the recorded A and B servo bursts, can come to be positioned entirely within the recording width of only one of the A and B bursts (outside the linear zone) rather than being positioned (inside the linear zone) so as to overlay portions of both. The magnitude of one of the detected A and B burst signals is a constant zero under such a condition and the difference between the amplitudes of the A and B burst signals cannot be used to produce an error signal that is linearly related to the actual distance (position error) between transducer position and track center. The condition introduces an undesirable nonlinearity into the transfer function of the track centering servo loop. This nonlinearity makes it difficult to minimize track centering time and the overall track seeking/centering speed of the disk drive is disadvantageously slowed as a consequence.
U.S. Pat. No. 4,669,004, entitled "High Capacity Disk File With Embedded Sector Servo" and issued May 26, 1987 to Moon et al., discloses one solution to this problem. Instead of using just the A and B bursts for each track, a quadrature approach is taken wherein four servo bursts, namely, A, B, C and D are recorded for each track. The advantages of the quadrature approach are discussed in U.S. Pat. No. 4,669,004, whose disclosure is incorporated herein by reference, and they do not have to be repeated here.
While the approach of U.S. Pat. No. 4,669,004 solves the problem of generating nonlinearities in the operating characteristics of a track centering servo, the approach has several disadvantages. Generation of the C and D servo bursts complicates the disk formatting process. The C and D servo bursts disadvantageously increase servo-data length and thus consume media space which could otherwise be used for storing nonservo data (user data). And the complex design of the quadrature based servo system tends to disadvantageously increase the cost of the disk drive. A need exists for a simpler solution.